1. Field of the Invention
This invention in general describes a reflectometer and in particular a reflectometer employing an integrating sphere and lens mirror concentrator.
2. Description of Related Art
Integrating spheres are used to measure diffuse reflectance and diffuse transmittance (also known as directional hemispherical reflectance/transmittance). In the visible spectrum, the integrating sphere coatings usually consist of a white diffuser (e.g., MgF, BASO.sub.4, or "Halon.RTM."). While for the infrared spectrum, an aluminum surface is frequently sandblasted and then plated with gold.
One of the chief disadvantages of integrating spheres, especially in the infrared spectrum, is that their throughput is low. Low throughputs lead to unacceptable signal-to-noise ratios when measuring samples with low reflectances or transmittances (e.g., &lt;1%). This is also related to the detector field-of-view
Recently, in Snail and Hanssen, Integrating Sphere designs with isotropic throughput, Appl. Opt. Vol 28, No. 10, May 15. 1989, pp. 1793-1799, a family of three reflectometer designs using nonimaging concentrators to restrict FOV of an integrating sphere's detector, with no concomitant loss in signal, were described. The designs exhibited a uniform throughput over the hemisphere above the sample, but each design had limitations. The first of the described designs is felt to be impractically long for small detector fields-of-view, whereas the second design, which utilized a compound elliptic concentrator (CEC) to view exactly one-half of the sphere, is very sensitive to misalignment errors with specular samples. The third design, which uses an inverted compound parabolic concentrator (CPC), is felt to be impracticable with dewared detectors in the infrared spectrum and also exhibits a nonuniform throughput in the visible light spectrum with a non-dewared detector if the reflectance of the CPC mirror is not sufficiently high (e.g., &gt;95%).